Since organic solvent series coatings discharge to the atmosphere their diluting organic solvent upon being used, they are regarded as one of important factors of global environmental problem. As a countermeasure therefor, water is employed as the diluting agent instead of the organic solvent, and emulsion coating materials emulsified through an emulsifying agent are employed.
For the emulsion coating material, ones of acrylics, urethanes and acryl silicones will form a main current. However, the coating of such emulsion coating material is deteriorated in the weatherability. Accordingly, a provision of an emulsion coating material which can form a film excellent in the weatherability has been demanded.
As the coating basis capable of forming the coating excellent in the weatherability, a reactive silicone compound having at molecular end alkoxide radical or silanol radical has been known. However, the reactive silicone compound could have been present stably within the organic solvent but has been difficult to be kept emulsified for a long term even when the compound is emulsified with water and an emulsifying agent added, since the alkoxide radical or silanol radical of the reactive silicone compound reacts with water for condensation polymerization so as to apt to cause a gelation, precipitation or the like to occur. Due to this, it has been required to control the reactivity of the reactive silicone compound with respect to water, in order to attain the stable emulsification for a long term of the reactive silicone compound.
Accordingly, ones in which silicone oil controlled in the reactivity to water by closing the molecular ends is employed as a coating base material are forming the main current of the silicone emulsion coating material. Since silicone oil is inferior in the reactivity, however, the crosslinking reaction within the coated film is difficult to advance, and any highly durable silicone coating cannot be obtained. Due to this, the silicone emulsion coating material the coating base material of which is silicone oil is mainly employed as a surface treatment material in the field of fabrics only, and cannot be employed as the coating material in the field of coating.
In WPI Acc Publication No. 95-363696/199547, for example, there has been suggested a three-dimensional, crosslinkable, water-dispersed silicone resin composition consisting of (A) polyorganosiloxane resin particulate matter of a softening point 50-250.degree. C., (B) water, (C) an emulsifying agent and (D) an emulsion of a specific polyorganosiloxane containing in 1 molecule at least one of hydroxyl radical or hydrolyzable radical. The particulate matter of polyorganosiloxane employed in this composition is, while not specifically limited, the reactive silicone having silanol radical in the molecule. However, its shape is particulate as being solid, and its molecular weight should preferably be more than 5,000 (Embodiment in the above publication), and the reactivity of the silanol radical with respect to water is considered low. Due to this, it is considered that a stable water-dispersed silicone is enabled. In order to form a hardened coating of this composition, however, it is required to cause to occur a condensation reaction of the silanol radical which is low in the reactivity, and a heat treatment at a high temperature of more than 200.degree. C. is required therefor, so as to be unable to cope with conditions for low temperature hardening. Further, a manufacturing method for this known compound has involved complicated works, in requiring steps of finely crushing and dispersing the particulate matter and so on.
In order to render the silicone emulsion coating material using a reactive silicone compound as the coating base material to be able to cope with the low temperature hardening conditions, there has been suggested in Japanese Patent Laid-Open Publication No. 58-101153 a method for rendering a silicone emulsion coating material to be hardened at room temperatures by an addition of such hardening catalyst as octylic acid tin or the like to the material.
The silicone emulsion composition disclosed in this publication is a three-solution mixture type silicone emulsion composition consisting of (a) a silicone emulsion of organosiloxane partial hydrolyzate having in 1 molecule more than two of silanol radicals, anionic emulsifying agent and water, (b) a uniformly dispersed solution of a reaction product of amino functional silane or its hydrolyzate with an acid anhydride and coloidal silica, and (c) a hardening catalyst. When applied to a substrate, this composition forms a coating having a rubber elasticity mainly according to two-dimensional crosslinking.
The above organosiloxane partial hydrolyzate employed in the composition is a sort of the reactive silicone compound and, while not limited, its molecular weight is disclosed to be more than 10,000 desirably (page 3, right column, line 6-8 of the publication). In case of such large molecular weight, a proportion in molecule of the silanol radical which participates in the reaction with water is almost zero, and the reactivity of organosiloxane partial hydrolyzate with respect to water is low. Because of this, the stability as an emulsion is considered to be seemingly high.
In the silicone emulsion composition disclosed in the above publication, however, the hardening catalyst is an essential component since no hardening occurs without the hardening catalyst. When organosiloxane partial hydrolyzate is made to coexist with the hardening catalyst, the crosslinking reaction of organosiloxane partial hydrolyzate is apt to progress. Due to this, such inexpediences as a gellation of the emulsion, white turbidity of the hardened coating and the like are apt to occur, and also there is a problem that costs are made high by the use of the hardening catalyst.